The present invention relates to a printed product color misregister amount detection method and apparatus for obtaining a color positional shift amount of a printed product printed by a multi-color printing press as a color misregister amount.
Conventionally, as a method of correcting a color positional shift of a printed product printed by a multi-color printing press, the following color registration methods (1) to (5) are available.    (1) A triangular register mark having an oblique side forming an angle of 45° with respect to a side perpendicular to the sheet convey direction is provided for each color. The misregister amount in the circumferential direction (sheet convey direction) is obtained from the distances among the perpendicular sides of the register marks of the respective colors. The misregister amount in the lateral direction (the direction perpendicular to the circumferential direction) is obtained from the distances among the oblique sides each forming an angle of 45° with respect to the perpendicular side of the corresponding color. The color positional shift amount among the respective colors is then corrected. This method is described in, e.g., Japanese Patent Laid-Open Nos. 62-39241, 62-231755, and 62-234934.    (2) A square register mark having a side parallel to a side perpendicular to the sheet convey direction is provided for each color. The misregister amount in the circumferential direction is obtained from the distances among the perpendicular sides of the register marks of the respective colors, and the misregister amount in the lateral direction is obtained from the distances among the parallel sides. The color positional shift amount among the respective colors is then corrected. This method is described in, e.g., Japanese Patent Laid-Open No. 3-15554.    (3) A circular dot-like register mark is provided for each color. The register marks of the respective colors are captured by a camera at once. The misregister amounts in the circumferential direction and lateral direction are obtained from the distances among the center positions of the register marks of the respective colors. The color positional shift amount among the respective colors is then corrected. This method is described in, e.g., U.S. Pat. No. 5,018,213.    (4) A crisscross register mark is provided for each color employed for the purpose of visual check adjustment. The register marks of the respective colors are captured by a camera at once. The misregister amounts in the circumferential direction and lateral direction are obtained from the distances among the center positions of the register marks of the respective colors. The color positional shift amount among the respective colors is then corrected. This method is described in, e.g., Japanese Patent Laid-Open No. 62-99149.    (5) A plurality of band-like reference-color register marks are printed at constant intervals in the circumferential direction and lateral direction. Band-like register marks of colors other than the reference color are printed to overlap the second and subsequent reference-color register marks of the respective directions. The register marks of the respective colors are captured by a camera at once and binarized. The misregister amount in the circumferential direction is obtained from the lengths of the respective binarized register marks in the circumferential direction and from the distance from the first register mark. The misregister amount in the lateral direction is obtained from the lengths of the respective binarized register marks in the lateral direction and from the distance from the first register mark. The color positional shift amount among the respective colors is then corrected. This method is described in, e.g., Japanese Patent Laid-Open No. 3-15553.
With the conventional color registration apparatus described above, the misregister amount is obtained from: the distances among the perpendicular sides of the register marks of the respective colors and the distances among the oblique sides each forming an angle of 45° with respect to the corresponding perpendicular side according to the method (1); the distances among the respective sides of the register marks of the respective colors according to the method (2); the distances among the center positions of the register marks of the respective colors according to the method (3); the distances among the center positions of the register marks of the respective colors according to the method (4); and the lengths of the respective register marks and the distance from the first register mark according to the method (5). Thus, a detection resolution equal to or more than the allowable misregister (e.g., 0.01 mm) is required. When capturing the images of the register marks by the camera at once, a high-accuracy, high-resolution camera must be used, leading to an expensive apparatus.
In the method (1), the misregister amount is obtained from the distances among the perpendicular sides of the register marks of the respective colors and the distances among the oblique sides each forming an angle of 45° with respect to the corresponding perpendicular side. In the method (2), the misregister amount is obtained from the distances among the respective sides of the register marks of the respective colors. During printing, as the printed portion tends to become thicker than intended, accurate detection cannot be performed. Also, if the misregister amount is to be obtained during conveyance of the printed product, as in the method (1), high-accuracy conveyance is required. An apparatus that satisfies this requirement becomes expensive.